Are solid-state batteries safer?
At first glance, solid-state batteries offer several safety advantages. They eliminate the flammable liquid electrolyte and can replace it with a non-flammable solid electrolyte. They also typically have a wider operating temperature range, making thermal runaway from cell overheating potentially less likely. Heat generated by an external heating failure is also usually reduced.
However, this doesn't tell the whole story. The term solid-state battery actually refers to a range of battery technologies. In some cases, the battery will still use a liquid component for ion exchange (semi-solid state), meaning there is still a volatile component. Some solid-state polymer electrolytes will not be completely flammable, and any electrolyte can melt if the system gets hot enough.
In 2022, the Paris public transport operator temporarily withdrew 149 electric buses after two separate fires. It is alleged that the cells used in these incidents employed batteries with an LFP cathode, a lithium metal anode, and a solid-state polymer electrolyte. The supplier describes its batteries as "completely solid, with no liquid components, and free of nickel and cobalt.".
Another example comes from a simulation-based research study conducted by Sandia National Laboratories in 2022 (Hewson et al., Joule, Vol. 6, Issue 4, 742–755) that compared the safety of a fully solid-state battery, a solid-state battery with added liquid electrolyte at the cathode, and a conventional liquid-based lithium-ion battery. The study found that, in the event of failure due to external heating, a solid-state battery with a small amount of liquid electrolyte generates less heat than a typical lithium-ion battery, but more than a fully solid-state battery. In the event of a short circuit, the heat released depended only on the cell capacity. Since solid-state batteries can have a higher energy density, more heat could be generated. Typical thermal runaway temperatures for standard lithium-ion batteries are around 1000–1200°C. In some scenarios of this research, the temperature increase of the solid-state batteries reached almost 1800ºC.
The development of solid-state batteries is ongoing, but the conclusion is that they can be safer in most cases. Even so, no battery system will be 100% safe. Therefore, thermal management and fire protection materials will always be necessary to provide that final layer that delays the spread of fire beyond the battery.
Fire Protection Material Solutions:
The types of fire protection materials used for solid-state batteries will be largely similar to those used for traditional lithium-ion batteries; the cell form factor (cylindrical, prismatic, pouch) and the overall pack design will have a greater impact on material selection. Currently, the most commonly used materials for passive fire protection include mica sheets, ceramic blankets, encapsulating foams, and flame-retardant coatings, among others. Aerogels are gaining market share, and options such as intumescent coatings and phase-change materials are receiving increasing interest.
Many of these materials would struggle to withstand temperatures above 1500°C. Even so, the ultimate goal is not necessarily to completely stop the spread of fire, but rather to delay it for as long as possible. In addition to high-temperature performance, these materials increasingly have to address other requirements, such as cell conformability, compaction performance, and cost. The rapidly growing electric vehicle market, increasingly focused on fire safety, will present diverse opportunities for fire protection materials, opportunities that will not be eliminated by alternative battery technologies such as solid-state batteries.
Author: Dr. James Edmondson, Principal Technology Analyst at IDTechEx
